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You can already use a U2F key with a Google account, such as a Yubikey 4 or other generic key.

Only with Chrome. Safari is a no go, for example.

Also, Yubikey U2F (e.g. Yubikey Neo) doesn't work on iOS devices [1].

This is actually the reason I switched back to Android for my primary phone -- complete lack of auth token support via USB or NFC.